Fix XML escape in JUnit formatter

[seikichi]

The JUnit formatter does not escape special characters correctly.
For example, consider the following (broken) lua program foo.lua.

local foo =

When I run luacheck with --formatter=JUnit option,

% luacheck --formatter=JUnit foo.lua
<?xml version="1.0" encoding="UTF-8"?>
<testsuite name="Luacheck report" tests="1">
    <testcase name="foo.lua:1" classname="foo.lua">
        <failure type="E011" message="foo.lua:2:1: expected expression near <eof>"/>
    </testcase>
</testsuite>

The above result is an invalid XML, the xmllint command reports following errors.

% xmllint <(luacheck --formatter=JUnit foo.lua)
/dev/fd/11:4: parser error : Unescaped '<' not allowed in attributes values
        <failure type="E011" message="foo.lua:2:1: expected expression near <eof
...

This commit fixes the following escaping problem.

[mpeterv]

Thank you! I'll release a bugfix version this week.

[mpeterv]

0.15.1 released, sorry for the delay 😞